Autosomal Dominant Polycystic Kidney Disease (ADPKD) is caused by mutations in PKD1or PKD2, the genes encoding polycystin-1 (PCI) and polycystin-2 (PC2), respectively. A cardinal feature of ADPKDis the formation of multiple fluid-filled cysts in both kidneys that progressively replace normal renal parenchyma, leading to renal failure. ADPKD is also associated with significant extrarenal manifestations; major among them is a vasculopathy, which can be life threatening. The growth and expansion of kidney cysts appears to require a somatic second hit affecting the normal PKD1 or PKD2 allele, but stochastic fluctuations in expression of the normal PKD allele below a critical level may also contribute to disease onset or progression. The factors involved in regulating PKDtranscript levels are poorly understood. In yeast two hybrid screens using the cytoplasmic tail of PCI (PC1CT) as bait, we have identified the adaptor proteins 14-3-3 as specific PCI interacting partners. Microarray expression profiling of E12.5 kidney-derived mRNA from our Pkdl knockout mouse embryos revealed that the 14-3-3 binding protein TAZ is significantly upregulated compared to wild-type littermates. TAZ shuttles between the plasma membrane and the nucleus, where it functions as a transcriptional co-activator or represser. We found that knockdown of TAZ in zebrafish causes pronephric cysts in addition to cardiomyopathy, abnormal axis curvature and fin-, neural tube- and myotome defects. The level of the PKDl-like transcript issignificantly reduced in the TAZ morphants. Taken together these findings suggest that TAZ is a potential transcriptional regulator of the PKD1 gene. Consistently, PKD1 cRNA completely rescued the cyst phenotype as well as the above neuromuscular defects in zebrafish TAZ morphants, indicative of a functional association between TAZ and PKD1in these tissues. An interaction of TAZ with the PKD1 gene product PCI could take place in the plasma membrane, through their common 14-3-3 partner, sequestering TAZ from the nucleus, thus limiting its transcriptional activity on PKD1 gene. The proteolyticallycleaved PC1CT could also move to the nucleus and modify the activity of TAZ in that compartment. These new findings will be further explored in two specific aims. 1) Map the functional domains in TAZ required to rescue the cystic phenotype in vivo using gene-replacement assays and determine its mode of interaction with PCI. 2) Identify the downstream transcription factors involved in TAZ regulation of PKDl. Biochemical, genetic, genomic and proteomic approaches will be utilized. The results from these studies will help define a novel pathway regulating the expression level of PKDl and hence contributing to the renal and extrarenal manifestationsofADPKD.